1. Field of the Invention
The present invention relates to an active matrix liquid crystal display using thin film transistors as drivers, and more specifically to an electrode structure of such a liquid crystal display for providing a high contrast ratio.
2. Description of Related Art
At present, active matrix liquid crystal displays are being rapidly used in wide and various fields because they enables a large scale of display and because they can drive a dot-matrix type color display with a low voltage.
Referring to FIG. 7, there is shown a diagrammatical sectional view of one typical conventional active matrix color liquid crystal display having thin film transistors (called "TFT" hereinafter) for selectively driving associated liquid crystal elements. The shown liquid crystal display has a substrate 701 having gate electrodes 702 connected to a scan line (not shown) and covered with an insulator layer 703. On each portion of the insulator layer 703 covering the the gate electrode 702, there is formed a TFT 704 having a drain electrode 705 and a source electrode 706, and on the other portion of the insulator layer 703, pixel electrodes 707 are provided one in association to each of the TFTs 704. Each of the pixel electrodes 707 is connected to the source electrode 706 of the associated TFT 704. Each of the drain electrodes is connected to a corresponding signal line (not shown) substantially diagonal to the gate electrodes 702.
A common counter electrode 724 is provided in opposite to the pixel electrodes 707 and separately from the pixel electrodes 707. In addition, a liquid crystal 725 such as twist-nematic liquid crystal is sandwiched between the pixel electrodes 707 and the common counter electrode 724, with an insulating layer being interposed between the pixel electrodes 707 and the liquid crystal 725 and another insulating layer being interposed between the common counter electrode 724 and the liquid crystal 725. Therefore, each of the pixel electrodes 707 defines one liquid crystal element. In correspondence to each liquid crystal element, one color filter such as a red filter 721, a green filter 722 or a blue filter 723 is located on the common counter electrode 724 so that the color filters form a mosaic pattern, and a color filter substrate 720 is provided to cover all the color filters 721, 722 and 723.
With the above mentioned arrangement, a voltage is selectively applied to the drain electrode through the signal line and the gate electrode through the scan line so as to turn on a TFT selected by an activated signal line and an activated scan line. As a result, a voltage is applied between the pixel electrode 707 and the counter electrode 724 of the liquid crystal element connected to the turned-on TFT, so that a light transparency condition of the liquid crystal between the pixel electrode 707 and the counter electrode 724 is changed under influence of the applied electric field, for example from an opaque condition to a transparent condition.
However, the actual liquid crystal display has 240 or 480 gate electrodes 702, which are sequentially scanned to be selectively applied with a voltage. Namely, each of the gate electrodes 702 is applied with no voltage for a time other than a selected time of period for driving. On the other hand, in most cases, each of the signal lines connected to the drain electrodes 705 is ceaselessly applied with a voltage unless the whole of one vertical line corresponding to the signal line should be black. If no voltage is applied to the gate electrode 702 of each TFT 704, the TFT will not be turned on, and therefore, no voltage is applied between the pixel electrode 707 and the counter electrode 724, but in most case a voltage is ceaselessly applied between the drain electrode 705 and the counter electrode 724.
As a result, an area in proximity of the drain electrode 705 and the signal line for supplying a video signal to the drain electrode 705 is ceaselessly applied with an electric field, as indicated by arrows in FIG. 7, and therefore, a portion of the liquid crystal element adjacent to the electric field applied area will consequently allow transparency of an error light as illustrated by Reference Numeral 750 in FIG. 7, other than the liquid crystal elements on the pixel electrodes selectively driven. Thus, the contrast ratio of each color and the purity of color are deteriorated, and chromaticity is decreased. As a result, an indication having a high saturation cannot be obtained.
Further, the signal line and the counter electrode are ceaselessly maintained in an electrostatically coupled condition. Therefore, an input voltage to the signal line is not sufficiently transmitted to the respective pixels. In order to resolve this problem, it is necessary to increase the drive voltage. This is contrary to the currently dominant demand for a low voltage driving.
Turning to FIG. 8, there is shown another sectional view of the conventional liquid crystal display taken along a line in parallel to a scan line (not shown) connected to the gate electrode (not shown). In FIG. 8, elements similar to those shown in FIG. 7 are given the same Reference Numerals. As shown by arrows in FIG. 8, an electric field is acted between an signal line 805 (connected to the drain electrode not shown in FIG. 8) and the counter electrode 724, so that an error transparent light 850 remarkably appears above and at opposite sides of the signal line 805 other than the TFT area. This will deteriorate the contrast not only in color liquid crystal displays but also in monochrome liquid crystal displays.
In order to resolve this problem in the case of color liquid crystal displays, there has been proposed to overlap all the color filters (red filter, green filter and blue filter) at a boundary area of adjacent pixels, as shown in FIG. 9. The overlapped portion of the three color filters will form a black matrix which defines the periphery of each pixel. But, this is not sufficient to shield the error transparent light. In addition, this has caused another problem in which the overlapped portion of the color filters will form a projection toward the substrate 701 with the result that the counter electrode 724 and the signal line (not shown) and the drain electrode 705 are strongly electrostatically coupled at the projected portion of the color filters as diagrammatically shown by Reference Numeral 960. This will make the low voltage drive more difficult.